2014-02-25

JunOS and ARP Glean

I'm using Cisco vocabulary 'glean' here as I don't know better word for it. Glean is any IPv4 packet which is going to connected host which is not resolved. It is NOT an ARP packet, so ARP policers won't help you. They are punted, since you need to generate ARP packet and try to resolve them.

In 7600 we can use 'mls rate-limit unicast cef glean 200 50' to limit how many packets per second are punted to control-plane for glean purposes. How can we limit this in JunOS? As far as I can see, there is no way. But I remember testing this attack and was unable to break MX80, so why didn't it break?

First let's check what does connected network look like

fisakytt@test13nqa1-re0.dk> show route forwarding-table destination 62.236.255.179/32 table default Routing table: default.inet Internet: Destination Type RtRef Next hop Type Index NhRef Netif 62.236.255.0/24 intf 0 rslv 828 1 xe-0/0/0.42

Ok, fair enough. Type 'rslv', which we can guess means packet is punted to control-plane for resolving ARP. Let's try to ping some address rapidly which does not resolve and check what it looks like

fisakytt@test13nqa1-re0.dk> show route forwarding-table destination 62.236.255.179/32 table default Routing table: default.inet Internet: Destination Type RtRef Next hop Type Index NhRef Netif 62.236.255.179/32 dest 0 62.236.255.179 hold 830 1 xe-0/0/0.42

Interesting, we get host/32 'hold' entry for it, no longer whole connected network as itself. Hold adjacency is programmed by software, so between 1st packet coming in and hold adjacency being programmed all packets in between are punted as well. I'm bit optimistic now that maybe there is some intelligence here. Maybe we'll get more information by checking the index (it changes quite often, so try to catch it at same time)

MX104-ABB-0(test13nqa1-re0.dk vty)# show nhdb id 830 extensive ID Type Interface Next Hop Addr Protocol Encap MTU Flags PFE internal Flags ----- -------- ------------- --------------- ---------- ------------ ---- ------------------ ------------------ 830 Hold xe-0/0/0.42 - IPv4 Ethernet 0 0x0000000000000000 0x0000000000000000 BFD Session Id: 0 Hold Status Packet: 0x4499d788 Timer: Running Flags: 0x0000000000000000

Well not too interesting. Didn't try to figure out what 'Packet' means yet, maybe it's memory location for packet which caused it to be on hold, maybe it's timer information. But curious thing is, that it's timed. So we'll know it'll go away automatically after some time. But how can we figure out what happens to hold and rslv? Maybe they are exceptions?

MX104-ABB-0(test13nqa1-re0.dk vty)# show jnh 0 exceptions terse Reason Type Packets Bytes ================================================================== Routing ---------------------- hold route DISC(70) 6339 532476 resolve route PUNT(33) 175 14700

Yup, the hold routes are discarded. I'm sending 10pps, so we're punting 0.27 per second. And we can extrapolate hold timer being about 3.7s. So 1 packet per 3.7s is punted. If I had been sending lot more packets, I'd see lot more punts, 10pps is just very slow rate so 'hold' adjacency will drop rest of them. Also if we have very many addresses this can add up to good number of packets. I might try to walk through the punt(33) to see if there is policer attached to it or not (There should be, but in certain situations it will be bypassed, this will be fixed and hopefully at same time we'll see ddos-policer limiters for resolve packets).

Just to be sure we've interpreted all this correctly, let's check that DISC(70) is seeing our ping packet to 62.236.255.179

MX104-ABB-0(test13nqa1-re0.dk vty)# debug jnh exceptions 70 discard MX104-ABB-0(test13nqa1-re0.dk vty)# debug jnh exceptions-trace MX104-ABB-0(test13nqa1-re0.dk vty)# show jnh exceptions-trace [430247] jnh_exception_packet_trace: ############### [430248] jnh_exception_packet_trace: [iif:338,code/info:198/0x273,score:(0x0),ptype:2/0,orig_ptype:2,offset:18,orig_offset:18,len:102] [430249] jnh_exception_packet_trace: 0x00: 20 00 c6 00 27 30 01 52 00 12 00 66 80 00 00 20 [430250] jnh_exception_packet_trace: 0x10: 12 00 00 12 00 10 00 00 00 3c 8a b0 8d a6 4a 28 [430251] jnh_exception_packet_trace: 0x20: c0 da 07 c1 c8 88 47 00 00 0d 3e 45 c0 00 54 06 [430252] jnh_exception_packet_trace: 0x30: 62 00 00 40 01 84 6d c1 58 ef 21 3e ec ff b3 08 [430253] jnh_exception_packet_trace: 0x40: 00 99 6b 3a c5 16 07 53 0c c7 ed 00 00 07 cb 08 [430254] jnh_exception_packet_trace: 0x50: 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 [430255] jnh_exception_packet_trace: 0x60: 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 [430256] jnh_exception_packet_trace: 0x70: 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37

3e ec ff b3 => 62.236.255.179, yup that's our ping packet being discarded due to 'hold' adjacency

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